This invention relates to a means for guiding a printed circuit board, which is to be tested, into precise location on a test fixture. More particularly, the invention includes a vacuum distribution medium for aligning and contacting an array of spring-loaded test probes (also called "pogo probes", and the array is sometimes called a "bed of nails") with a printed circuit board to be tested.
The electronics industry uses bed of nails test fixtures for the electrical testing of printed circuit boards, both bare and loaded circuit boards. Such test fixtures contact a circuit board with an array of test probes, so that the test probes electrically contact predetermined test points on the board. The operator then tests the entire circuit, or individual circuit components, using methods known in the art.
In commercially available test fixtures in present use, tooling guide pins fixed onto the test fixture position the printed circuit board to be tested over the test probe array. A printed circuit board usually has two or more tooling holes. The tooling guide pins locate in the board's tooling holes, and thus locate the board.
In order for the test results to be accurate, the test probes must accurately contact their corresponding test points on the board. As circuit and component density increases, the required degree of accuracy also increases.
A printed circuit board test system may have a large number of programmable signal drivers to input various stimulation signals to the printed circuit board to be tested, and a large number of programmable signal receivers for receiving the various signals generated by the printed circuit board as a result of the input stimuli. Typically, such a printed circuit board test system has an array of test probes arranged in a rectangular array or grid having 100 mil. centers. A particular printed circuit board to be tested will usually have a large number of test points which are randomly distributed over the surface of the printed circuit board.
Although existing test fixtures may use a vacuum actuated system for contacting a printed circuit board with an array of test probes, most of those systems are complex in structure, having a number of complicated moving parts.
The present invention addresses the above-noted and other drawbacks of the prior art by providing a test fixture to contact a printed circuit board with an array of test probes.
The invention provides a vacuum-operated pull-down device of uncomplicated construction having no moving parts apart from the spring-loaded test probes. The spring-loaded test probes line up substantially with each test point on the printed circuit board. The invention eliminates the undesirable feature of complex moving parts common to existing devices.
The test fixture includes a platen assembly. The platen assembly includes two substantially identical plates which when connected together form a vacuum chamber. One of the plates has a hole which serves as a vacuum port. The platen assembly has passageways extending through both plates. The passageways are located according to the template used to drill the plated-through holes in the printed circuit board under test. Tooling guide pins in vacuum-tight relation in two of these passageways locate with tooling holes on the printed circuit board. Test probes extend in vacuum-tight relation through other passageways. When an operator places a printed circuit board to be tested on the tooling guide pins, the plated-through holes located at the test points of the printed circuit board line up substantially with the test probes.
A peripheral seal on the surface of the platen forms an air-tight seal between the surface of the platen and the printed circuit board.
Generally, only about one-third of the passageways in the platen are required for test probes. The remaining passageways are left vacant. To enable the chamber to hold a vacuum, the operator seals with a sealing tape the vacant holes opening out to the surface of the platen opposite the printed circuit board. The operator then applies a vacuum to the chamber, and the remaining holes on the side of the platen facing the printed circuit board cause a vacuum in the cavity beneath the printed circuit board, lowering the board and causing the test points to contact their respective test probes.